将配置文件读入 Haskell 时发生 IO

Question

I have input data intended for my yet-to-be-written Haskell applications, which reside in a file. I don't update the file. I just need to read the file and feed it into my Haskell function which expects a list of strings. But reading the file of course yields IO data objects. I have learned that using the <- operation can "take out" somehow the strings packed in an IO structure, so I tried this attempt:

run :: [String]
run = do
  datadef_content <- readFile "play.txt" -- yields a String
  let datadef = lines datadef_content -- should be a [String]
  return datadef

I placed this into a file play.hs and loaded it from ghci by

:l play

To my surprise, I got the error message for the readFile line

 Couldn't match type 'IO' with '[]' Expected type: [String] Actual type: IO String

and for the return the error message

 Couldn't match type '[Char]' with 'Char' Expected type: [String] Actual type: [[String]]

The first seems to indicate that I couldn't get rid of the IO , and the last message seems to suggest, that lines would return a list of list of strings, which also doesn't make sense to me.

How can I do this correctly?

Answer 1

You declare run to be a [String] value. But return is not a keyword that provides the return value of a function; it is a function, with type Monad m => a -> ma . return datadef produces a value of type IO [String] , which becomes the return value of the function.

The solution is to provide the correct return type for run :

run :: IO [String]
run = do
    ...

run can also be defined more succinctly as

run = fmap lines (readFile "play.txt")

Though the do syntax suggests is, there is no way to pull a value out of an IO action; all you can do is "push" the call to lines into the action.